I come from VyOS and really liked it, but still prefer opnsense for the GUI, constant updates and plugins. VyOS started losing appeal once they opted for subscription stable iso access (even if they did give me a free subscription for some comment contribution in their repo). Also, I have to admit, that VyOS needs a fraction of the resources needed by opnsense.

I tried virtualizing Windows on proxmox and it went smooth

I am not saying your server is not secure, but just fencing off IMAP from the web is not enough to limit spammers from relaying mail through your server. They usually exploit a misconfigured SMTP server, which does run on port 25 (plain or start TLS mode)